If confidence could be bottled, it would be an overnight sensation. Confidence is a trait we admire in others and lament the absence of in ourselves. But while some people may indeed just be born confident, it’s a skill the rest of us can easily acquire.

As all the best performers and public speakers know, confidence is as much about appearance as it is about feeling it. So where better to start honing your confidence than in your voice, one of the most important...